In a small jar (2-cup capacity) measure 3 tablespoons bread flour and add 2 tablespoons water. Mix well.
Loosely cover jar with lid and place jar in a warm place in your kitchen. I put mine in the cupboard.
After 24 hours, take the jar, add 3 tablespoons of flour and 2 tablespoons of tepid water. Stir until well mixed.
Put the lid back loosely and return the jar in the same place.
By day 3 you will see bubbles forming...sign that there is some bacteria present. Congratulations. You may now start thinking of a name. :D
Keep feeding your starter for a week.
Day 7 - by this time you already have a young starter. Start discarding some of the starter before feeding to freshen the batch and make it stronger. Use a digital kitchen weighing scale. Put a bowl on top and make sure the scale is on "0"
Pour 100 grams of sourdough starter in the bowl then press the scale to "0" to measure the water. Or ⅔ cup flour and ½ cup water
Pour 100 grams of tepid water to the sourdough mixture in the bowl. Stir well.
Press the scale to "0". Add 100g of flour to the bowl and stir until it's well mixed.
Pour the contents of the bowl into a clean and bigger jar (1 quart capacity) to give more room for your sourdough to rise. Loosely cover the jar and keep it in a warm place Put a rubber band around the jar so you can keep track of it's growth. Repeat for a week. You will notice by the end of week 2 (day 14) that you have a happy, bubbly wild yeast in your care, ready for its first baking experience. At this time you may try using it for simple sourdough bread recipes if you like.
To continue, on Day 15, I started feeding my sourdough starter every 12 hours.
Continue to do this if you are baking almost everyday. After 6 weeks I noticed my sourdough starter is okay even if I missed a "feeding". It has a more sour smell when it is hungry then after feeding, it smells sweet again. As an insurance, in case I unintentionally "killed" my wild yeast, I kept an ounce of sourdough starter in the fridge 5 hours after feeding it an ounce of flour and one ounce water with the lid tighly closed. I plan to feed it once per week. That's it. You can now use your sourdough starter at this point to make delicious sourdough bread. Enjoy your new "baby"
Good Luck:)